Rust, glyphosate drift raising red flags on wheat

STARKVILLE, Miss. - What a few weeks ago looked to be a potentially high-yielding Delta wheat crop is now reeling from a one-two punch from rust disease and herbicide injury.

Wheat specialists throughout the Delta are reporting widespread damage caused by disease and off-target glyphosate applications. They say it’s too early to tell whether the crop is down and out or will recover before the referee finishes his nine-count.

“Substantial wheat acreage in Mississippi is showing symptoms of herbicide injury,” says Erick Larson, Extension wheat specialist at Mississippi State University in Starkville. “Although we do not yet have chemical analysis confirmation of glyphosate presence, our consensus belief is that glyphosate drift is most likely causing the symptomology apparent.

“The damage likely occurred in the days prior to the March 15 ban on aerial applications of burndown herbicides, but the symptomology is just now occurring.”

Larson says he’s received reports of widespread drift damage scattered throughout the Delta, but the majority of the cases reported are located south of Highway 82 in the central portion of the region.

“The cases I’ve seen appear to have received a sub-lethal dose of glyphosate, which is causing chlorosis or whitening of leaf tissue, often in an interveinal pattern, generally beginning near the leaf collar extending out towards the leaf tip. Most of the injured fields are exhibiting this chlorotic injury on the flag leaves. Some wheat heads are also often malformed, twisted or bent over when they have difficulty emerging from stunted leaf sheaths in the boot stage.”

Confirming the presence of similar injury to other grass species such as Johnsongrass or other relatively immature grass species outside of the field would help confirm presence of glyphosate drift, he says.

It may be two to three weeks before the potential damaging effects are known, but Larson says the resulting yield loss could be quite severe if the off-target glyphosate has damaged or sterilized the wheat’s reproductive organs. “Unfortunately, we will likely need some time to completely evaluate the extent of head damage until after flowering and kernel development begin.

“Of course, chlorosis will limit photosynthetic capacity of the flag leaf, which will cause yield reduction as well, but this will likely be relatively minor, compared to sterility problems.”

Because weather conditions in February delayed some burndown applications until early to mid-March, aerial applicators had only a few days to make those necessary applications before the Mississippi Bureau of Plant Industries’ burndown restrictions came into play in the Mid-Delta.

Larson says many of the drift complaints are likely to be attributed to aerial applications made slightly before the March 15 deadline set by the state.

The problems with glyphosate drift are also showing up in neighboring Arkansas. Jason Kelley, Extension wheat and feed grains specialist in Little Rock, says he’s afraid the drift damage is going to be more widespread than most people think.

Identifying cases of glyphosate damage in wheat has been difficult, he says, because in many cases the affected fields do not look damaged at first glance. “You can drive by these fields, and from the road they look fine, but that’s not the case. We’re going to see more cases of off-target herbicide damage as more wheat growers get out into the fields to take a closer look.”

Most of the suspected drift damage to the state’s wheat crop is being attributed to glyphosate applications made by aerial applicators between March 5 and March 10, according to preliminary investigations made by the state’s Plant Board.

“The glyphosate damage is causing chlorotic streaks in the flag leaf,” Kelley notes. “The flag leaves are also smaller than normal, a lot of the wheat heads that just started to head out are an off-color, and a portion of those wheat heads affected will more than likely be sterile. Also, a lot of wheat heads are coming out the side of the boot, causing crooked and misshapen heads.

“It’s mostly in southern Arkansas now, but it’s just a matter of time before it starts to show up in the northeast region of the state,” he says. “We had more than an average amount of burndown herbicides go out by airplane, simply because of the wet conditions early in the year.”

The wheat yield-damaging culprit in Louisiana is rust, according to Boyd Padgett, Extension plant pathologist and research scientist with the LSU AgCenter at the Macon Ridge Research Station in Winnsboro.

“Stripe rust is bad again this year in Louisiana,” said Padgett. “It’s a cool-season rust and with this unexpected cold front that we got recently, the lower nighttime temperatures could drive an epidemic. This is the fourth year that we’ve had stripe rust to any noticeable degree, and it looks like we’re stuck with it now.

“We’re still on our learning curve with this disease, but we do know it moves quickly and is a rather aggressive disease.”

Stripe rust requires both cool temperatures and moisture to flourish. It can proliferate even if daytime temperatures remain moderately warm and there is little to no rainfall, because cool nighttime temperatures with an accompanying dew is often enough to cultivate the rust spores. It is more likely to occur earlier in the season under cool, cloudy conditions with rainfall.

Wheat growers in Louisiana have also been reporting cases of leaf rust, a warmer weather disease. Unfortunately, this means that the state’s growers could be faced with treating both stripe rust and leaf rust in the same year.

Identifying which form of rust is present in your wheat field is relatively simple, says Mississippi State’s Larson. The pustules of stripe rust align themselves more parallel with the leaves, and are generally a yellow-orange color. In cases of leaf rust, random pustules that are a darker, medium to burnt orange color are often found covering much of the plant’s leaf tissue.

There is no “absolute” threshold growers can rely on to trigger fungicide treatments for rust, Padgett says. “Typically, if you’ve got a good yield potential and the only symptoms of rust are located on the flag leaf, you could justify putting a treatment out as long as the crop has not yet matured. If the wheat is heading out and flowering, you don’t need to make a fungicide application.”

Also worth considering is the disease resistance level of the variety planted, and whether or not the forecasted temperatures are favorable to further development of the disease. For instance, if cool nights are projected for the next 10 days Padgett says a fungicide treatment may be warranted to stripe rust.

“Stripe rust can function between 32 and 68 degrees F, but it prefers the 50-to 60-degree nights and nighttime dews we’ve had in recent weeks. If you’ve got a field at flag leaf emergence or the boot stage, good yield potential and an extended forecast that favors development of the disease, with a $4 per bushel wheat price, there’s no question treatment would be advisable,” Padgett says.

“There is no cut and dry threshold. It essentially boils down to the price of wheat, the susceptibility of the variety planted, what the extended weather forecast is projecting, and whether or not you’ve got a wheat crop worth protecting.”

While there have been scattered cases of rust in Mississippi this year, Larson says the numbers so far are not significant. That doesn’t mean that growers won’t be faced with the decision whether or not to treat for rust in the coming weeks.

“There are two or three widely planted wheat varieties in Mississippi that are fairly susceptible to rust, particularly stripe rust. In those varieties, we could fungicide applications could prove to be necessary in the coming weeks,” Larson says.

In Arkansas, wheat growers are making preventative fungicide sprays before rust has much of a chance to take hold in their fields.

Rick Cartwright, Extension disease specialist in Little Rock, Ark., says there are cases of stripe rust being reported in east central Arkansas, but the cool temperatures and fungicide treatments are holding the disease back for now. In the Red River region of the state, Cartwright says stripe rust is more active and more fungicide treatments are being made to wheat.

“A lot of preventive fungicide treatments are going out, and we’re not seeing a rust epidemic this year like we did in 2000. However, the weather conditions we’re experiencing do favor stripe rust, so we’re keeping an eye on it,” he says.

The presence of the disease does make a difference to a lot of growers deciding whether or not to treat their wheat with a fungicide, but it doesn’t make as much of a difference when wheat prices are high, Cartwright says.

That’s especially true for those growers locking in favorable pricing opportunities, and anticipating wheat yields of as much as 80 bushels per acre. “When you’ve got a history of rust, a high yield potential and $4 plus wheat prices, your willingness to invest in a late season input like a $15 an acre fungicide is much higher,” he says.

“Overall, I’m impressed by how clean the crop is, disease-wise. The wheat just looks fantastic. If this crop doesn’t set a record, I don’t know what will,” Cartwright adds.